The present invention relates to a manufacturing method of a liquid crystal display element with improved display quality as a result of prevention of decomposition or degradation of the liquid crystal, and also relates to a manufacturing device for use with the method.
A typical liquid crystal display element is manufactured in the following manner. Transparent electrodes are formed on the inner sides of a pair of transparent substrates. Then the transparent substrates are subjected to an alignment process. Sealing material is applied around display areas of the transparent substrates. Using the sealing material, the substrates are subsequently combined to form a liquid crystal cell, with an injection opening being left open to allow the charging (injection) of liquid crystal through it. Liquid crystal is then charged in the liquid crystal cell through the injection opening. After the charging, the injection opening is closed using a sealant made of a UV-curing resin composition. To finish the manufacture, ultraviolet light is projected to the sealant to cure it.
Various methods have been suggested to improve the display quality of a liquid crystal display element manufactured by the above manufacturing processes. For example, Japanese Laid-Open Patent Application No. 60-170830/1985 (Tokukaisho 60-170830; published on Sep. 4, 1985) discloses a method to manufacture a highly reliable liquid crystal display element by preventing the formation of foam inside the liquid crystal cell. According to the disclosure, the foam formation is prevented by a heating and evacuation process in which the liquid crystal cell, before being filled with liquid crystal, is heat-dried to remove residual water content inside the cell and evacuated to 10xe2x88x926 Pa to 10xe2x88x927 Pa (10xe2x88x925 Torr to 10xe2x88x926 Torr).
More specifically, as shown in the flow chart of FIG. 6, in the xe2x80x9cheatingxe2x80x9d step, the transparent substrate is heated at 130xc2x0 C. using a vacuum oven or the like. Water content is then removed by evacuating the cell to about 10xe2x88x926 Pa in the xe2x80x9cevacuationxe2x80x9d step. In the next step of xe2x80x9cair introduction,xe2x80x9d the air is introduced into a chamber of the slowly cooled vacuum oven to restore normal pressure. The transparent substrate is removed from the chamber to form transparent electrodes on it in the xe2x80x9ctransparent electrode formationxe2x80x9d step. A pair of transparent substrates are subjected to alignment, and sealing material is applied around the display areas of the pair of transparent substrates in the xe2x80x9cliquid crystal cell fabricationxe2x80x9d step. In the following step of xe2x80x9ccutting,xe2x80x9d liquid crystal cells, which are formed as a single piece by combining the two substrates using the sealing material with an injection opening being left open to allow the charging of liquid crystal, are now cut into many individual cells or a cell cluster in which cells are lined up end to end.
In the xe2x80x9cheatingxe2x80x9d step which immediately follows the cutting, the individual liquid crystal cell is heated at 130xc2x0 C. using a vacuum oven. Water content is then removed by evacuating the cell to about 10xe2x88x926 Pa in the xe2x80x9cevacuationxe2x80x9d step. In the next step of xe2x80x9cair introduction,xe2x80x9d the air is introduced into a chamber of the slowly cooled vacuum oven to restore normal pressure. The liquid crystal cell is then removed from the chamber. Thereafter, liquid crystal is charged through the injection opening in the xe2x80x9cinjectionxe2x80x9d step. To complete the fabrication of the liquid crystal display element, the injection opening is sealed with a sealant, which is then exposed to ultraviolet light to cure in the xe2x80x9csealingxe2x80x9d step.
When molecules in the liquid crystal absorb ultraviolet light in a particular wavelength region, they become optically excited and highly active, causing radical reactions and optical reactions. The excited liquid crystal molecules decompose or degrade (as a result of oxidization) due to interaction with the material composing the alignment film and reactions with foreign objects (for example, oxygen, water content, acid compounds such as nitrogen oxides and sulfur oxides, and gaseous and other molecules in solvents used in the manufacturing processes of the liquid crystal display element) that are found in the liquid crystal cell. To address these problems, for example, Japanese Laid-Open Patent Application No. 11-2825/1999 (Tokukaihei 11-2825; published on Jan. 6, 1999) discloses a method to manufacture a liquid crystal display device with excellent display quality through the prevention of decomposition and degradation of the injected liquid crystal. According to the disclosure, 300 nm or shorter wavelength light (electromagnetic waves) is removed from the ultraviolet light emitted from a light source, such as a high pressure mercury lamp or a metal halide lamp, and the remaining light is projected to cure the sealant.
A vacuum impregnation technique is typically used in the xe2x80x9cinjectionxe2x80x9d step described in the foregoing manufacturing processes of liquid crystal display element to charge liquid crystal to a liquid crystal cell. The device to implement such a vacuum impregnation technique is disclosed, for example, in Japanese Laid-Open Patent Application No. 11-287998/1999 (Tokukaihei 11-287998; published on Oct. 19, 1999). According to the disclosure, a liquid crystal injection device is constituted by three separate chambers: a preliminary heating chamber for performing preliminary heating on a liquid crystal cell, a preliminary defoam chamber for performing preliminary defoaming on liquid crystal, and a vacuum chamber. In the device, a liquid crystal cell and liquid crystal are introduced to the vacuum chamber after undergoing preliminary heating and preliminary defoaming respectively, so as to charge the liquid crystal to the liquid crystal cell.
However, according to the disclosure in Japanese Laid-Open Patent Application No. 60-170830/1985 mentioned earlier, the liquid crystal cell is heat-dried and evacuated before the depressurization is terminated to revert to normal pressure. Consequently, the air is allowed to enter the liquid crystal cell. Oxygen, water content, acid compounds such as nitrogen oxides and sulfur oxides including sulfurous compounds, and gaseous and other molecules in solvents used in the manufacturing processes of the liquid crystal display element, which are found in the air, are attracted to the alignment film inside the liquid crystal cell. The attracted molecules adversely affect the liquid crystal, causing it to change undesirably over time. The resultant liquid crystal display element fails to deliver a satisfactory level of display quality.
In addition, a general purpose vacuum pump is not powerful enough to evacuate the cell to 10xe2x88x926 Pa to 10xe2x88x927 Pa, i.e., is not sufficient to achieve an extremely high degree of vacuum. Therefore, performing steps in such a high degree of vacuum requires a special vacuum pump, such as a diffusion pump, turbo molecular pump, cryopump, or sputter ion pump, which is capable of creating an extremely high degree of vacuum.
The concentration extinction coefficient k of a liquid crystal is given by the equation:
k=[log(1/T)]/C 
where T is the light transmittance in percentage points (%) and C is the concentration in grams per liter (g/L). FIG. 7 shows that when the incident light wavelength exceeds about 330 nm (ultraviolet region of the spectrum), the concentration extinction coefficient k of a typical liquid crystal equals 0. Typical liquid crystal molecules have a light absorption threshold wavelength at about 330 nm; they absorb ultraviolet light in a 330 nm or shorter wavelength region and, in particular, absorb ultraviolet light in a 320 nm or shorter wavelength region extremely well. Meanwhile, the high pressure mercury lamp and the metal halide lamp, which provide popular light sources, emits ultraviolet light with a line spectrum at 313 nm. Therefore, the method disclosed in Japanese Laid-Open Patent Application No. 11-2825/1999 still falls short of completely removing ultraviolet light in such a wavelength region that prompts decomposition and degradation of liquid crystal, and causes the sealant to be exposed to light including UV light in an about 300 nm to about 330 nm wavelength region. With the manufacturing method disclosed in the Patent Application, the liquid crystal is adversely affected by ultraviolet light projected to the sealant to cure it, which results in decreases in the voltage retaining ratio. It is thus difficult to manufacture a liquid crystal display device with excellent display quality.
If the liquid crystal interposed between electrodes degrades, an electric current flows through the liquid crystal from one electrode to the other. The electric flow reduces the voltage retaining ratio and makes it impossible to keep a voltage level across the electrodes which is required to cause liquid crystal to produce electro-optical effects. The liquid crystal display element cannot be turned on in these circumstances. Especially, if only a particular part of the liquid crystal cell is exposed to ultraviolet light, the voltage retaining ratio drops there, and the liquid crystal display element displays an image with burns.
Commercially available ultraviolet ray filters made of glass are used very often to obstruct the ultraviolet light which is intended to cure the sealant from being projected to the liquid crystal, and eventually, to prevent deficient displays detailed above. To mass produce liquid crystal display elements, liquid crystal needs to be charged to many liquid crystal cells at a time, as well as the sealant applied to those many cells needs to be cured collectively. To implement this, ultraviolet light inevitably needs to be emitted to cover a large area in a single projection. Problems lie where a glass-made ultraviolet ray filter covering a large area is costly and likely to be damaged by an impact, for example. Therefore, it is not desirable from an industrial point of view to use a filter to prevent the liquid crystal display element from developing display deficiencies.
In the device disclosed in Japanese Laid-Open Patent Application No. 11-287998/1999, water content and various solvent gas molecules are attracted and stick to the surface of the alignment film in the liquid crystal cell during its manufacture, which can be only insufficiently removed. The attracted molecules adversely affect the liquid crystal, causing it to change undesirably over time. The resultant liquid crystal display element fails to produce a uniform display and fails to deliver a satisfactory level of display quality.
An objective of the invention of the present invention is to present a method and a manufacturing device used with the method, whereby the voltage retaining ratio of the liquid crystal is kept unchanged through the prevention of decomposition or degradation of the liquid crystal, and a liquid crystal display element is thereby manufactured with improved display quality.
To achieve the objective, a manufacturing method of a liquid crystal display element in accordance with the present invention, including the sequential steps of:
(1) evacuating a liquid crystal cell by means of depressurization;
(2) substituting an inert gas for contents of the liquid crystal cell; and
(3) charging liquid crystal to the liquid crystal cell.
With the arrangement, foreign objects (for example, oxygen, water content, acid compounds such as nitrogen oxides and sulfur oxides, and gaseous and other molecules in solvents used in the manufacturing processes of the liquid crystal display element) are removed from the interior of the liquid crystal cell, and there is no risk of the air being charged in the liquid crystal cell. Therefore, there is no risk of the liquid crystal charged in the liquid crystal cell degrading due to adverse effects of the foreign objects. Accordingly, the voltage retaining ratio is kept unchanged in the liquid crystal. A liquid crystal display element is thus manufactured with improved display quality. Further, since an inert gas is substituted for the contents of the liquid crystal cell. So, a specialized vacuum pump (such as the one dedicated to create an extremely high degree of vacuum) is no longer necessarily used in the evacuation of the liquid crystal cell. A general purpose vacuum pump serves the purpose sufficiently. This further facilitates the manufacture of the liquid crystal display element.
To achieve the objective, a manufacturing device of a liquid crystal display element in accordance with the present invention includes:
a depressurization device for depressurizing a liquid crystal cell;
an inert gas substitution device for substituting an inert gas for contents of the liquid crystal cell; and
a liquid crystal charging device for charging liquid crystal in the liquid crystal cell,
the three devices being lined in this order with no intervening members.
With the arrangement, foreign objects (for example, oxygen, water content, acid compounds such as nitrogen oxides and sulfur oxides, and gaseous and other molecules in solvents used in the manufacturing processes of the liquid crystal display element) are removed from the interior of the liquid crystal cell, and there is no risk of the air being charged in the liquid crystal cell. Therefore, there is no risk of the liquid crystal charged in the liquid crystal cell degrading due to adverse effects of the foreign objects. Accordingly, the voltage retaining ratio is kept unchanged in the liquid crystal. A manufacturing device is thus realized which is capable of manufacturing a liquid crystal display element with improved display quality. Further, since an inert gas is substituted for the contents of the liquid crystal cell. So, a specialized vacuum pump (such as the one dedicated to create an extremely high degree of vacuum) is no longer necessarily used in the evacuation of the liquid crystal cell. A general purpose vacuum pump serves the purpose sufficiently. This further facilitates the manufacture of the liquid crystal display element. In addition, since the three devices are lined with no intervening members, steps can be implemented without a break from the evacuation of the liquid crystal cell to the charging of liquid crystal in the liquid crystal cell.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, are not in any way intended to limit the scope of the claims of the present invention.